1. Field of the Invention
This invention relates to an injector for supplying a jet of fuel into e.g. an internal combustion engine in an automobile, its movable core and a method of manufacturing the same.
2. Description of the Prior Art
A conventional fuel injector has a needle housed movably in a body having at its bottom a valve seat on which the needle normally rests. If an electric current is supplied to a solenoid coil, it attracts the needle away from the valve seat and forms therebetween a clearance through which fuel flows. The fuel is injected through a fuel injection port at the bottom of the body. The injection of fuel is continued as long as the supply of the electric current is continued, and if the supply of the electric current is discontinued, the needle returns on the valve seat again to terminate the injection of fuel.
The parts composing the injector are required to be highly accurately made to ensure the accuracy of fuel injection. The known injector includes a movable core defining at its top a seat for a return spring adapted to close the fuel injection port. The repeated use of the injector is, however, likely to bring about an undesirable change in the load of the return spring and thereby a lowering in the accuracy of fuel injection.
The movable core defines a fuel passage therein, and is, therefore, required to have an accurate surface finish. While it has been usual to form such a core by turning from a bar, cold forging has come to be considered as a more efficient method for core manufacture.
It is, however, likely that a block of material from which a movable core is formed by cold forging may have microcracks in its cut surface, and that those microcracks may form fine flakes on the surface of the core as forged. Those flakes are likely to come off the core to some extent or other and close the fuel injection port. Japanese Patent Application Laid-Open (KOKAl) No. Hei 1-166844 proposes the removal of such flakes by shaving or grinding the cut surface of a block of material prior to cold forging. The proposed method is, however, likely to bring about a lowering of productivity and a rise in the cost of manufacture as new problems, though it may effectively overcome the problem of flakes.
Another problem is the likelihood of the cut surface of a block of material to be seized with a die (including a die and a punch) during cold forging.